1. Field of the invention
The present invention concerns apparatus for reading optically a scene made up of a set of symbols of any kind on a transparent medium.
2. Description of the prior art
In numerous fields of activity it is necessary to process sets of graphical or other signs (texts, drawings, photographs, etc) that can be read from an opaque or transparent flat medium such as a sheet of paper or a plastics material film, for example. The processing to which these sets of signs are usually subjected in essentially directed to digitizing the image, to modifying their presentation and/or to deriving specific information from them.
In printing, for example, this type of processing is typically used for formatting text and illustrations during the process of designing books and newspapers. With regard to the latter, and with regard to daily newspapers in particular, the laying out of the pages imposes specific constraints in that it has to be done quickly and has to be modifiable virtually instantaneously to allow for newsworthy events occurring just before the paper is printed.
Another area in which this type of processing is routinely applied is the manufacture of printed circuits. Printed circuit boards are generally produced by chemical etching of exposed parts of a thin layer of copper bonded onto an insulative material and covered with a photo-sensitive film. The copper is exposed through a photographic mask consisting of a transparent film on which the conductive (or insulative) parts of the circuit to be manufactured are represented by opaque areas. If for any reason it is required to manufacture printed circuits featuring variations or modifications as compared with a printed circuit for which the mask is available, a new mask is produced from the original mask, which is therefore subjected to the kind of processing generally referred to above.
Until relatively recently, in order to modify or amend a document where the medium was a sheet of any material the work was done directly on the medium. At present, for reasons of speed and reliability, use is usually made of computer-controlled machines, the facilities of which include the possibility of working without any regard to the physical medium. As computers can process only digital data, the computer processing of a document is necessarily preceded by the digitization of the signs constituting it. This digitization is applied after the acquisition of said signs in the form of analog data by optical sensor devices.
At present two types of optical sensor device are mainly employed for acquiring signs readable on a flat medium. Hereinafter the term "scene" will be used to designate the combination consisting of the readable signs and their medium.
A first optical acquisition device which is suitable for reading documents on a flexible medium comprises a drum onto which the scene to be acquired is fixed, for example by means of a suction system producing a reduced pressure inside the drum. Parallel to the axis of the drum is a straight guide rail along which can move a laser beam emitter/ receiver. In operation the drum rotates at constant speed. The emitter/receiver moves at constant speed along the rail and the laser beam, the end of which describes on the drum a spiral with contiguous turns, is modulated by the signs on the sheet attached to the drum. This device has the disadvantage of being able to read only documents on a flexible medium and of having a relatively limited acquisition rate on the order of 400 kHz). It is not possible to increase the rotation speed of the drum beyond a threshold at which it is deformed due to centrifugal force (the so-called "barrel" effect).
A second type of device for optical acquisition of a flat scene comprises a plate movable parallel to an X-X' axis, above which is a fixed rail guiding a carriage that moves parallel to a Y-Y' axis, the X-X' and Y-Y' axes being mutually perpendicular. The scene to be acquired is attached to the plate. The carriage carries an optical acquisition system comprising a sensor, an objective lens whereby the sensor is focused on the scene and a light source for illuminating the part of the scene read by the sensor. The sensors used are routinely of the charge-coupled device (CCD) linear type (meaning that they are made up of photo-sensitive elements disposed along a single line), or matrix type (meaning that they are made up of photo-sensitive elements disposed in m rows and n columns). Linear sensors have the advantage over matrix sensors of being less costly and of enabling the scene to be scanned at constant speed. Devices of this second type, and in particular those comprising a linear sensor, have a very high acquisition rate (in the order of 20 MHz) and can read with great fidelity all types of documents, including those on a rigid medium (glass, for example). It is obvious that this speed and this fidelity result from the use of mechanical systems (for guiding and driving the plate and the carriage, for example) and optical systems (objective lens, etc) which are both very accurate and very highly specified in terms of performance, and which are consequently extremely costly. These devices have the further disadvantage of being relatively bulky and of requiring numerous and frequent adjustments.
An object of the present invention is to provide apparatus for reading optically documents on a transparent medium which has an acquisition rate. comparable with that of existing linear sensor devices but which is compact, of relatively simple construction, very much less costly than prior art devices, requires absolutely no adjustment in use and offers extremely precise magnification.